The present invention relates to surface mount components and more particularly to a method for extending the life expectancy of surface mount components by improving the reliability of a solder joint.
Most electronic assemblies include surface mount components such as chip capacitors, chip resistors or the like. Standard, or conventional components are readily available at a very low cost, but with only average performance reliability. In the majority of applications, such as automotive applications, it is necessary to maintain a high level of performance reliability and consequently a long life expectancy for the surface mount components.
An alternative to standard components that increases the reliability of the component are specialized components whose performance is proven more reliable than standard components. The specialized components have compliant leads on the ends that induce lower strains on the solder joints as compared to components without leads, thereby improving the reliability. However, these components are manufactured using highly specialized processes, adding to the cost of the component and ultimately to the cost of the electronic assembly.
Another alternative is to improve the reliability of the solder joint, which ultimately translates into a more reliable surface mount component. This is because in surface mount designs, the height of the solder joint between the bottom of the surface mount component and the solder pad, known as the stand-off height, is an area subject to relatively high stresses under operating conditions. The stresses are mainly due to the difference in the thermal expansion of the surface mount component and the thermal expansion of the printed circuit board, which contributes to solder joint failure. An improved solder joint would allow a standard component to operate more reliably and on the same level as a specialized component. Up until now, the focus for improving solder joint reliability has been on modifying the shape of the solder joint.
One method of improving the solder joint reliability is to prevent solder from entering the area between the component and the pad. This design is difficult to manufacture and, therefore, extremely costly. Another method, discussed in U.S. Pat. No. 5,639,013, is to form a convex solder joint thereby increasing the stand-off height. In this method, a predetermined amount of solder is applied to the solder pads on a circuit board. A predetermined amount of solder is placed between the component and the solder pads. The solder is melted so that the component floats to a predetermined height above the circuit board and forms a convex shape.
The disadvantage of this method is that a convex solder joint is an unnatural shape for a solder joint and is extremely difficult to produce reliably. Often times during the manufacturing process of a convex solder joint, the surface mount component exhibits an undersirable effect known as xe2x80x9ctombstoningxe2x80x9d. This undesirable effect is created by excess solder that causes the surface mount component to rotate vertically by 90 degrees and stand upright on one of the solder pads, mimicking a tombstone.
There is a need for a simple, reliable method of increasing the stand-off height of a surface mount component without the need for specialized components, unnatural solder joint shapes and excess solder.
The present invention is a method of increasing the life expectancy of surface mount components used in electronic assemblies. In particular, the life expectancy of a surface mount component can be significantly lengthened by increasing a stand-off height of the component from the printed circuit board. The stand-off height is defined as the distance between the bottom of the surface mount component and the printed circuit board. The stand-off height is controlled through the manufacturing process used to attach the surface mount component to the printed circuit board by inserting a spacer to set the stand-off height before soldering the component to the printed circuit board.
It has been determined through computer simulation that increasing the stand-off height considerably increases the life expectancy of the surface mount component. The method of the present invention improves the life expectancy of surface mount components, thereby improving the reliability of the component in harsher operating environments. In addition, a significant cost savings is realized because it is possible to replace specialized components with standard components and improve the reliability of the electronic assembly. For example, conventional low-cost chip resistors can replace expensive wire-wound resistors and still maintain high reliability of the electronic assembly.
It is an object of the present invention to provide a method for improving the life expectancy of surface mount components used in electronic assemblies. It is another object of the present invention to provide a significant cost saving to the cost of the electronic assembly. It is still another object of the present invention to use standard surface mount components and realize highly reliable electronic assemblies.
Other objects and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims.